This invention relates to an extrusion-type coating apparatus, especially to a coating apparatus that allows trouble-free coating in manufacturing magnetic recording media. A similar apparatus is disclosed in copending U.S. patent application Ser. No. 07/297,756 of Seiichi Tobisawa et al., entitled Coating Apparatus, filed on Jan. 20, 1988.
A variety of coating processes are available; these include roll coating, gravure coating, extrusion coating, slide-bead coating, and curtain coating.
Magnetic recording media are available by coating magnetic coat solutions commonly by means of roll coating, gravure coating, and extrusion coating. Among these, extrusion coating is preferable since it provides uniform coat-film thicknesses.
The magnetic recording medium itself, however, has improved rapidly in recent years; its material consists of an oxidized magnetic powder which has a high BET value, and barium ferrite. High viscosity coating solutions are also being increasingly used. For the purpose of higher productivity, high-speed coating is in growing demand.
Conventional techniques in the extrusion coating process intended for manufacture of the magnetic recording medium is described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 84771/1982, No. 104666/1983, and No. 238179/1985.
In fact, said extrusion coating process provides uniform coat film thicknesses, but only under good coating conditions limited to a narrow range; said extrusion coating process, however, often fails to attain desired coating under the high viscosity, high-speed coating conditions described above.
Although a coating head with a specially designed shape enables high-speed coating at a rate of 300 m/min or more with a thick film thickness, said extrusion coating process cannot provide a marketable magnetic recording medium because of uneven thickness, greatly out of tolerance, which is inevitable under high-speed conditions.
Under these types of coating conditions, thin film (30 .mu.m or less before drying) coating in particular, said extrusion coating process causes failures which lead to major problems. For example, a coating film is partially peeled off by foreign matter, dust, coagulating particles, etc. which are deposited on the base material and transferred to or caught by the back edge face, the film thickness is partially thickened, and the base material is shaved by the front edge, especially the corner at the downstream end of the front edge, and the resulting base waste is deposited. In particular, a high viscosity coating is likely to cause a crosswise streak failure as a result of the stick-skip motion of the base material, it also causes noise and fluctuations of output.
Various countermeasures are taken for the above failures. A typical example is the technique disclosed in Japanese Patent O.P.I. Publication No. 238179/1985 (hereinafter referred to as prior technique).
The prior technique is derived from the technique described in Japanese Patent O.P.I Publication No. 104666/1983. In this technique the back-edge face 2' has a triangular cross section, as shown in a hypothetical line in FIG. 9. This is to prevent foreign matter from overriding the triangular cross section, and the foreign matter thus blocked is likely to deposit in the solution sump P thereby causing a streak failure. In the prior technique, not only the back-edge face 2, is smoothed as shown in a solid line in FIG. 9, but .THETA..sub.1 and .THETA..sub.2 are so designed as to satisfy the conditions of formula (1). EQU .THETA..sub.1 &lt;.THETA..sub.2 &lt;180.degree. (1)
In fact, the prior technique effectively reduces the incidence of failures in that no foreign matter is left in the solution sump P, but it is still subject to the failure caused by waste from the base material whose surface is shaved by the downstream end B of the front edge face 1', this is where contact pressure between the base material and downstream end B is concentrated because the running angle of the base material changes abruptly.
On the other hand, in thick-film coating, the base waste failure is reduced to a certain extent because a contact pressure of the base material to the downstream end B is partly distributed to the back-edge face, as compared with thin-film coating. The coating film thickness is solely dependent on the balance between the pressure of the coating solution on the back-edge face, and the pressing force of the base material on the back edge face during running. This balance is so fine that the slightest unbalance results in stepped unevenness which means thicknesses differ along the running direction of the base material, or streaks along the running direction and with different thicknesses in the crosswise direction. The higher the coating speed, and the thicker the film thickness, the greater is the incidence of coating failure.
The first aim of the invention is to provide a coating apparatus that allows excellent coating under conditions of a thick film thickness, and high-speed coating without causing the stepped unevenness and streaks.
FIG. 9 shows the prior technique. Air is included between the front face 1' and the surface of the base material while the base material is running, and is blocked at the downstream end B of front face 1' whereby air is not allowed to enter the coating solution. However, this design causes base waste to deposit. The present inventors have found that a contact force of the base material is effectively distributed to the back face by projecting at least a part of the back face from the tangent line l.sub.1 at the downstream end B of the front face. As the back face is projected, however, a contact force between the downstream end B and the base material is weakened accordingly. In particular, when the base material runs at a rather high rate, more boundary air on the base material surface flows over the downstream end B so that the air is not securely blocked, with the result that pinhole failures on the sheet are likely to occur after coating.
The base material applies pressure to the front face and back face and the components of the forces are then balanced with each other, as stated above. The forces and their balance or distribution of components have so delicate an influence on coating properties that a coating head with an optimum shape is involved. However, there is a limitation on shape design for improving coating properties. For example, if coater heads are changed each time a base material with a different specification flows, productivity is reduced.
In addition, it is extremely difficult to eliminate uneven thicknesses of the base material in the cross-wise direction, because a different tension acts on the base material in that direction.
The second aim of the invention is to provide a coating apparatus that successfully prevents coating failures such as uneven film thicknesses and allows coating conditions to be changed easily.
The coater head is generally made of stainless steel.
However, when a new roll of the base material is charged or any trouble occurs during coating, and the base material 1 is separated from the coater head CH as shown in FIG. 6, the coating solution A' drips down along the surface of front edge or front face 1 from slit 2. For this reason it is common practice to clean front edge 1 with a solvent before the coating apparatus is restarted.
However, insufficient cleaning or an especially extended shutdown time causes the coating solution to solidify or for dry waste to remain on front edge 1, and waste is coated on the base material after coating is restarted, causing a coating failure. The coating failure is diminished within several minutes, during which the high speed coating produces a failure length of several hundreds meters from the restart position, causing reduction in productivity (yield).
The third aim of the invention is to provide a coating apparatus that successfully prevents coating failures caused by waste of the coating solution which remains on the edge face of the coater head, and that ensures an improved yield.
In addition to the extrusion-type coating apparatus as described above, the present invention discloses another method of manufacturing magnetic recording media while adding a curing agent in-line.
Japanese Patent O.P.I Publication No. 10773/1983 discloses a method for in-line addition of a curing agent, when performing roll coating with a magnetic coating solution, for creating higher stability of the magnetic coating solution and improving characteristic such as squareness ratio.
In fact, the method of in-line addition of a curing agent is an excellent method of improving stability of a magnetic coating solution. More particularly, if a curing agent is added before the magnetic coating solution is conditioned, the curing agent reacts with the binding agent in the solution tank, solution viscosity rises with time, and coating properties depart from the tolerance range, with the result that no magnetic recording medium is manufactured to the required quality level unless the magnetic coating solution is finely re-conditioned each time when required. In fact, the in-line addition method is thoroughly free from such trouble, thereby improving the stability of the coating solution.
However, it should be noted that the in-line addition method has been employed mainly by the coating apparatus that applies a coating solution kept in a vat such as for gravure coating, as described in Japanese Patent O.P.I. Publication No. 10773/1983. More particularly, the coating solution in the coating solution supply line is not totally applied on the base material but only a part of the solution is applied, with the residue left in the vat. A residual solution in the vat causes the reaction to progress in the vat, the viscosity of the coating solution to rise with time, and the coating solution partially become highly viscous, with the result that the stability of the coating solution is insufficiently maintained.
In particular, when a binding agent has a polar group, the performance of the tape is greatly deteriorated because of the high thixotropy of the coating solution.
The fourth aim of the invention is to provide a method of manufacturing magnetic recording media with excellent characteristics by improving the stability of the coating solution.
The magnetic tape, which is a magnetic recording medium, has a back-coating layer formed on the surface of a base support material which is opposite to the magnetic layer surface, whether through an uncoated layer or not. This is done to improve the running stability and to reduce the friction resistance between tapes.
After the magnetic layer is coated, the back coating layer is commonly coated by means of gravure coating, reverse coating, or kiss-roll coating.
However, when the back-coating layer is formed by conventional means after the magnetic layer is coated, the magnetic tape drops out because the magnetic layer is nipped by the rubber roll while the back coating layer is coated, dirt deposited on the roll or contained in the coating solution is pressed to the magnetic layer to produce blemishes or scratches. In recent years, magnetic recording media with lower dropouts are in growing demand, and require an improved coating method.
The fifth aim of the invention is to provide a method of manufacturing magnetic recording media by forming a backcoating layer in a series of coating processes while reducing dropouts.